By Sinead Carew and Jessica Toonkel

Sept 26 (Reuters) - U.S. broadcasters that typically can count on a big revenue bounce from political advertising in presidential election years are still waiting for that to happen.

Heading into the first presidential debate on Monday night, Republican nominee Donald Trump has been relying far more on free media than past candidates, and some broadcasters already say they have taken a hit.

Last week, Sinclair Broadcast Group, with a presence in 81 markets, said its political ad revenue could be about a third lower than its forecast. Gray Television, with stations in 50 markets, rescinded its third-quarter estimate of $40 million to $46 million in political ad revenue, citing limited visibility in the "very unusual" election.

In August, E.W. Scripps Co. warned that its political ad revenue could fall more than 10 percent short of its original $150 million estimate.

Investors and analysts are bracing for more disappointment.

"It's not going to be peaches and cream," said Bill Smead, chief investment officer of Seattle-based Smead Capital Management, whose Smead Value fund owns shares in broadcaster Tegna Inc.

Broadcasters' fortunes still have time to improve. Over the past two election cycles, about 40 percent of the year's political ad spending took place in October, said Steve Passwaiter, vice president and general manager of Kantar Media's CMAG unit.

But the patterns of the 2016 election cycle suggest that political ad spending could trail prior years.

Through August, Trump spent just $28 million on TV ads, while Democratic nominee Hillary Clinton spent $133 million. Overall, the candidates and political action committees spent $1.1 billion on local TV ads through August, compared with $2.9 billion for all of 2012, according to Kantar CMAG.

Trump's campaign in recent days has indicated it would spend another $100 million on TV ads, including $60 million on regional ad spots, ahead of the Nov. 8 election, according to an Associated Press report. Campaign representatives did not respond to a request for comment.

"It's not a great number," said Benchmark Co. media analyst Daniel Kurnos, referring to Trump's plans.

HIGH MARGINS

Presidential races typically account for about 30 percent of political ad revenue, according to E.W. Scripps spokeswoman Carolyn Micheli. At Scripps, she said, political ads carry the highest profit margins, up to 80 percent, compared to overall industry margins of 30 percent to 40 percent.

TV companies' share of political ad revenue depends largely on whether their stations are located in battleground states.

Sinclair, whose market valuation has declined 16 percent this year, and Gray Television, whose stock has fallen 38 percent, both have stations in Virginia, for example, which is typically a swing state.

But this year, with Senator Tim Kaine of Virginia the vice presidential running mate of Clinton, the state has attracted far fewer ad dollars than expected, according to Sinclair.

Some broadcasters are sending more positive signals. CBS Corp Chief Executive Les Moonves said last week that the network's political ad revenue was on track, though he cited strong spending on senatorial and congressional races while admitting that Trump's spending has been low.

Tribune Media Co has not changed its $200 million target for the year, though at a Sept. 15 investor conference, Chief Executive Peter Liguori talked about uncertainty around the "Trump factor."

Nexstar Broadcasting Group Inc, whose stock has fallen 8 percent this year, said it already has bookings for two thirds of its 2016 $100 million target, and it reaffirmed that target.

"Presidential's not going to drive our political spend. It's going to be Senate races and gubernatorial races," said Nexstar's chief executive, Perry Sook. The group has stations in 18 states with Senate elections.

Local broadcasters' shares fell as much as 9 percent on Sept. 19, when Sinclair issued its warning, and they have yet to recover fully. For the year, broadcaster stocks are down 8 percent to 38 percent.

(Additional reporting by Michelle Conlin; Editing by Eric Effron and Leslie Adler)